JUPITER, Fla. — With the emergence of so many young pitchers, Jon Niese has become the Mets forgotten starter, and he may not be in line for Opening Day.

Don’t underestimate the importance of Niese, who finally had his first spring training start of the season Tuesday in a 9-8 victory over the Cardinals at Roger Dean Stadium.

The numbers were ugly, but Niese’s shoulder held up in his two-inning stint as he faced 14 batters, surrendered six hits, four runs and walked two. He did not strike out a batter and allowed one home run. Niese topped out at 89 mph. He threw 50 pitches.

His first fastball was clocked at 79 mph.

“When the first inning started, after about the third pitch I was a little worried,’’ manager Terry Collins said. “But he settled down and pitched OK.’’

In the second inning Niese gave up four runs.

It’s up to Niese to see if he will be the Opening Day starter. It’s 19 days until the first game.

“We won’t know until he goes out there a couple more times,’’ Collins said of Niese and Opening Day. “We have to make sure we got the right guy that can give us the innings we need on Opening Day for sure.’’

Asked if the Opening Day starter is up in the air still, Collins said, “Yeah, it is right now.’’

If not Niese, the Opening Day starter figures to be Bartolo Colon, who won 18 games for the A’s last season, or Dillon Gee, who has pitched well against the Nationals.

“[Pitching coach] Dan [Warthen] has it set up that if it’s not Jon, it’s going to be Dillon or most likely Colon, depending on how they throw in the next two weeks,’’ Collins said.

Colon started Monday, which puts him on the perfect five-day track to start Opening Day.

The Mets are veteran-geared for that Opening Day start and are not considering Zack Wheeler, for now.

As Niese is concerned, all that mattered was his health.

“My shoulder feels like 100 percent,” Niese said. “It doesn’t feel weak at all, but a lot of velocity has to do with my body being on time and staying closed, just using my whole body instead of my arm. I think that will come. My arm feels better and better each time, that’s a good sign.’’

Niese, 27, must stay healthy this season. It’s time.

It’s time for Niese to step up and lead this Mets staff. Matt Harvey is out. Wheeler is entering his first full season. Colon is a newcomer. Gee is just starting to find his changeup groove again.

“I just want to stay healthy and hit that 200-inning mark,’’ Niese said. “By getting to the seventh inning each outing, that will give my team a chance to win. The results will come.’’

Niese was given that five-year, $25.5 million contract extension in 2012. He has the stuff, when healthy.

Niese has not been told anything about Opening Day. He does know there is much work to be done.

“I don’t feel like my pitches are crisp,’’ Niese said. “That has something to do with mechanics.’’

Kolten Wong blasted a two-run home run in the Cardinals’ four-run second on the pitch after Niese threw one under his chin. The home-run pitch to Wong, Niese said, “was a [crummy] cutter. I felt that I released in the back of my head. Poorly executed.

“I’m getting better, the more I throw off the mound, the better I feel, the more mentally it’s going to be behind me,’’ he said of his shoulder issue. Earlier this spring, Niese told Collins his arm felt “dead.’’

As Niese came off the baking field at Roger Dean Stadium, he said, “Man, it was hot out there.’’

It’s going to get hotter. This is the season Jon Niese must produce for the Mets in a big way.

Would Niese be your pick for opening day?

It's the same story with Niese year after year: flashes of brilliance followed by injury, followed by struggling to regain his confidence, at which point the cycle begins all over again. Put it all together and you have a modest 43-40 career which accurately reflects his contributions.

Gee has earned a shot at the opener as has Colon on the basis of his excellence last season. Let Niese find himself away from the spotlight...again.